1. Field of the Invention
The present invention relates to a photosensitive copper paste used for forming a desired electrode pattern on a substrate surface or each of the substrates which constitute a multilayer substrate in manufacturing a circuit board or a multilayer substrate, or the like, and a method of forming a copper pattern using the photosensitive copper paste.
2. Description of the Related Art
Radio-frequency electronic parts used for mobile communication equipment, satellite receivers, computers, etc. have been increasingly miniaturized in recent years and concurrently undergone an increase in performance with miniaturization of these apparatuses. Also, wiring patterns of the radio-frequency electronic parts have strongly been required to support an increase in density and signal transmission speed.
In order to achieve increases in the density and the signal transmission speed of the wiring patterns of the radio-frequency electronic parts, the wiring patterns must be made fine while increasing their thickness (thickening).
A wiring pattern of a radio-frequency electronic part is conventionally formed by a method comprising forming a pattern on an insulating substrate by using a conductor paste containing a conductive metal powder composed of a polyvalent metal such as copper, and an organic vehicle comprising an organic binder and an organic solvent, and then drying the pattern and baking the pattern to form a predetermined wiring pattern.
Although a screen printing method is generally used for forming the wiring pattern, this method makes it difficult to decrease the wiring width and the wiring pitch of the wiring pattern to 50 xcexcm or less in order to obtain a fine wiring pattern. It is generally recognized that a wiring width and wiring pitch of about 50 xcexcm or less each are the refining limit of the screen printing method.
On the other hand, a photolithography method of forming fine and thick wiring by using a photosensitive conductor paste is proposed in Japanese Unexamined Patent Application Publication Nos. 5-287221 and 8-227153. This method comprises coating, on an insulating substrate, a photosensitive conductor paste comprising a conductive metal powder, an acrylic copolymer having carboxyl groups and ethylenic unsaturated groups in the side chains thereof, a photoreactive compound and a photopolymerization initiator, drying the coating, and then patterning the coating by photolithography.
Also, a photolithography method of forming fine and thick wiring by using a photosensitive conductor paste containing a glass powder is proposed in Japanese Unexamined Patent Application Publication Nos. 6-224538 and 8-335757. In this method, the glass powder is contained in the photosensitive conductor paste to improve adhesion between the conductor pattern and the ceramic substrate.
In consideration of the environment, the photolithography method using a photosensitive conductor paste has recently been desired to be developed with water or an alkali aqueous solution. Therefore, an acid functional group having the property of liberating protons, such as a carboxyl group or the like, is introduced into the organic binder. However, when a polyvalent metal, particularly copper, is selected as a conductor for the photosensitive conductor paste, the copper ions elute and react with the organic binder anions produced after release of protons to form a three-dimensional network due to ion crosslinkage, thereby causing gelation. The gelation of the photosensitive copper paste causes the problems of difficulties in coating and destabilizing development even if coating can be performed.
As a method of preventing gelation, for example, Japanese Unexamined Patent Application Publication No. 9-218509 discloses a photosensitive conductor paste containing, as a gelation inhibitor, a phosphorus-containing compound such as phosphoric acid; Japanese Unexamined Patent Application Publication No. 9-218508 discloses a photosensitive conductor paste containing a compound having an azole structure, such as benzotriazole; Japanese Unexamined Patent Application Publication No. 9-222723 discloses a photosensitive conductor paste containing an organic compound having a carboxyl group, such as acetic acid. However, the methods of using the gelation inhibitor can only slightly lengthen the time to gelation of the photosensitive copper paste, but difficulties in use of the photosensitive copper paste remain under the present conditions.
Also, in Japanese Unexamined Patent Application Publication No. 10-171107, 3-methyl-3-methoxybutanol is used as an organic solvent for preventing gelation. However, a phenomenon similar to gelation, i.e., a phenomenon in which a three-dimensional network is formed by ion crosslinkage to increase the substantial molecular weight, occurs in the dry paste, thereby causing the problem of failing to elute an unexposed portion with a developer.
The present invention has been achieved for solving the above problems, and an object of the present invention is to provide a photosensitive copper paste causing less gelation, and exhibiting excellent storage stability and permitting the formation of a fine and thick copper pattern having high adhesion to a substrate. Another object of the present invention is to provide a method of forming a copper pattern, a circuit board and a ceramic multilayer substrate using the photosensitive copper paste.
As a result of various experiments and research performed for achieving the above objects, the inventor found that by using a copper powder having a surface coating of a copper oxide in a system containing an organic binder having an acid functional group and the copper powder, gelation can effectively be inhibited.
Further experiment and examination led to the achievement of the present invention.
A photosensitive copper paste of the present invention comprises a mixture of an organic binder having an acid functional group, a copper powder and a photosensitive organic component, wherein the copper powder comprises a copper oxide coating on the surface thereof, and at least the surface layer having a thickness of about 0.1 xcexcm from the surface being composed of CuO as a main component.
The photosensitive copper paste of the present invention comprises the copper powder having the surface coating of copper oxide in which at least the surface layer having a thickness of about 0.1 xcexcm from the surface is composed of CuO as a main component. Therefore, the occurrence of gelation can be sufficiently suppressed either in the paste state before coating or in the coated state after coating and drying. Therefore, the photosensitive copper paste can be coated, patterned by exposure and developed, and then baked to efficiently form a fine and thick copper pattern.
In the photosensitive copper paste of the present invention, the organic binder having an acid functional group is a wide concept representing an organic binder comprising a material having a functional group having the property of releasing protons, such as a: carboxyl group, a hydroxyl group, a sulfonic group or the like, or an organic binder containing a material having the functional group. The type of the acid function group is not limited.
The photosensitive copper paste of the present invention comprises the copper powder having the surface coating composed of CuO as a main component at least in the surface layer having a thickness of about 0.1 xcexcm from the surface thereof. This is because with the surface layer composed of CuO as a main component and having a thickness of about 0.1 xcexcm or less, a region (inner layer) composed of Cu2O as a main component readily occurs in the outermost layer of the copper powder during kneading in the process for producing the photosensitive copper paste.
In the present invention, xe2x80x9ccomposed of CuO as a main componentxe2x80x9d is a concept representing that the molar ratio of CuO exceeds about 50%. The copper powder preferably has an average particle diameter of about 1 to 10 xcexcm, and the surface layer composed of CuO as a main component is preferably about 0.1 xcexcm to 1 xcexcm.
The photosensitive organic component used in the photosensitive copper paste of the present invention is a conventional known photopolymerizable or photomodifiable compound. Examples of such a compound include the following:
(1) A mixture of a monomer or oligomer having a reactive functional group such as an unsaturated group or the like, and a photo-radical generating agent such as an aromatic carbonyl compound;
(2) A diazo resin such as a condensation product of aromatic bisazide and formaldehyde; and
(3) A mixture of an addition polymerizable compound such as an epoxy compound, and a photo-acid generator such as a diallyl iodonium salt or the like; and
(4) A naphthoquinone diazide compound.
Of these photosensitive organic components, the mixture of a monomer or oligomer having a reactive functional group such as an unsaturated group or the like, and a photo-radical generator such as an aromatic carbonyl compound is particularly preferred.
Examples of the photo-radical generator include benzyl, benzoin ethyl ether, benzoin isobutyl ether, benzoin isopropyl ether, benzophenone, benzoylbenzoic acid, methyl benzoylbenzoate, 4-benzoyl-4xe2x80x2-methyldiphenylsulfide, benzyl dimethyl ketal, 2-n-butoxy-4-dimethyl aminobenzoate, 2-chlorothioxanthone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, isopropylthioxanthone, 2-dimethylaminoethyl benzoate, ethyl p-dimethylaminobenzoate, isoamyl p-dimethylaminobenzoate, 3,3xe2x80x2-dimethyl-4-methoxybenzophenone, 2,4-dimethylthioxanthone, 1-(4-dodecylphenyl-2-hydroxy-2-methylpropane-1-one, 2,2-dimethoxy-1,2-diphenylethane-1-one, hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1-phenylpropane-1-one, 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-one, methylbenzoylformate, 1-phenyl-1,2-propanedione-2-(o-ethoxycarbonyl) oxime, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-1-butanone, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, and the like.
Examples of the monomer or oligomer having a reactive function group include hexanediol triacrylate, tripropylene glycol triacrylate, trimethylolpropane triacrylate, stearyl acrylate, tetrahydrofurfuryl acrylate, lauryl acrylate, 2-phenoxyethyl acrylate, isodecyl acrylate, isooctyl acrylate, dodecyl acrylate, caprolactone acrylate, ethoxynonylphenol acrylate, 1,3-butanediol diacrylate, 1,4-butanediol diacrylate, diethylene glycol diacrylate, tetraethylene glycol diacrylate, triethylene glycol diacrylate, ethoxy bisphenol A diacrylate, propoxyneopentyl glycol diacrylate, tris(2-hydroxyethyl)isocyanurate triacrylate, ethoxy trimethylolpropane triacrylate, pentaerythritol triacrylate, propoxy trimethylolpropane, triacrylate, propoxy glyceryl triacrylate, pentaerythritol tetraacrylate, ditrimethylolpropane tetraacrylate, dipentaerythritol hydroxypentaacrylate, ethoxy pentaerythritol tetraacrylate, tetrahydrofurfuryl methacrylate, cyclohexyl methacrylate, isodecyl methacrylate, lauryl methacrylate, triethylene glycol dimethacrylate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,4-butanediol dimethacrylate, diethylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, ethoxy bixphenol A dimethacrylate, trimethylolpropane trimethacrylate, and the like.
The photosensitive copper paste of the present invention preferably further contains an ultraviolet absorber. By mixing the ultraviolet absorber, the absorptivity of exposure light can be improved, and exposure failure due to light scattering can be minimized. As the ultraviolet absorber, for example, an azo red pigment, an amine red dye or the like can be used.
The photosensitive copper paste of the present invention may further contain an inorganic component such as a glass powder, a ceramic powder or the like in order to improve adhesion to the substrate. A known glass powder such as borosilicate glass or the like can be used as the glass powder, and a known low-temperature sintering ceramic powder such as alumina, zirconia, crystallized glass ceramic, glass composite ceramic, non-glass ceramic or the like can be used as the ceramic powder.
When the inorganic additive such as the glass powder or ceramic powder contains a polyvalent metal compound, the polyvalent metal may be at least one selected from the group consisting of boron, lead, zinc, bismuth, aluminum, magnesium, calcium, barium, titanium, strontium, zirconium, manganese, cobalt, nickel, iron, yttrium, niobium, lanthanum and ruthenium.
Examples of the glass powder include powders containing oxides of polyvalent metals having a valence of two or more, such as a SiO2xe2x80x94PbO system, a SiO2xe2x80x94ZnO system, a SiO2xe2x80x94Bi2O3 system, a SiO2xe2x80x94K2O system, a SiO2xe2x80x94Na2O system, a SiO2xe2x80x94PbOxe2x80x94B2O3 system, a SiO2xe2x80x94ZnOxe2x80x94B2O3 system, a SiO2xe2x80x94Bi2O3xe2x80x94B2O3 system, a SiO2xe2x80x94K2Oxe2x80x94B2O3 system, a SiO2xe2x80x94Na2Oxe2x80x94B2O3 system and the like.
Examples of the ceramic powder include powders containing compounds of polyvalent metals having a valence of two or more, such as oxides, borides, nitrides and suicides of at least one polyvalent metal selected from the group consisting of lead, zinc, aluminum, magnesium, calcium, barium, strontium, zirconium, manganese, cobalt, nickel, iron, yttrium, lanthanum and ruthenium.
With the inorganic additive containing a polyvalent metal component, gelation occurs due to reaction with the acid functional group of the organic binder. In order to prevent reaction between the polyvalent metal component in the inorganic additive and the acid functional group of the organic binder, it is effective to add at least one of the following four additives:
(1) Anion-adsorbing material
(2) Thixotropic agent
(3) Alcohol having a boiling point of 178xc2x0 C. or more
(4) Fatty acid amide
The anion-adsorbing material (1) may have the form of inorganic fine particles or organic fine particles. As the inorganic fine particles, hydroxyapatite, hydrotalcite, zirconium phosphate, hydrous antimony oxide and the like are preferably used. As the organic fine particles, an anion exchange resin or the like can be used. Examples of the anion exchange resin include the following:
1. A resin comprising a matrix copolymer of divinylbenzene and acrylate, methacrylate or acrylonitrile, in which a primary, secondary, tertiary or quaternary amino group is incorporated as an ion exchange group;
2. A resin comprising a matrix copolymer of trivinylbenzene and acrylate, methacrylate or acrylonitrile, in which a primary, secondary, tertiary or quaternary amino group is incorporated as an ion exchange group;
3. A resin comprising a matrix copolymer of trimethylolpropane trimethacrylate ester and acrylate, methacrylate or acrylonitrile, in which a primary, secondary, tertiary or quaternary amino group is incorporated as an ion exchange group; and
4. A resin comprising a matrix copolymer of ethylene glycol dimethacrylate ester and acrylate, methacrylate or acrylonitrile, in which a primary, secondary, tertiary or quaternary amino group is incorporated as an ion exchange group.
As the thixotropic agent (2), an agent generally referred to as a xe2x80x9cthickening, sagging and sedimentation inhibitorxe2x80x9d or xe2x80x9csagging and sedimentation inhibitorxe2x80x9d, or xe2x80x9cpigment wetting, dispersion and sedimentation inhibitorxe2x80x9d can be used.
As the xe2x80x9cthickening, sagging and sedimentation inhibitorxe2x80x9d, a vegetable polymerized oil system, a polyether-ester surfactant, a hydrogenated castor oil system, a mixture of a hydrogenated castor oil system and an amide system, a fatty acid amide wax system or the like can be used.
As the xe2x80x9csagging and sedimentation inhibitorxe2x80x9d, a special fatty acid system, a sulfate ester system, an anionic surfactant, a polyethylene oxide system, a mixture of a polyethylene oxide system and amide system or the like can be used.
As the xe2x80x9cpigment wetting, dispersion and sedimentation inhibitorxe2x80x9d, a fatty acid polyvalent carboxylic acid, a amine salt of high-molecular weight polyester, a polyether-ester anionic surfactant, a long-chain amine salt of high-molecular-weight polycarboxylic acid, a salt of long-chain polyaminoamide and high-molecular acid polyester, a salt of long-chain polyaminoamide and phosphoric acid, a special modified polyamide system, a phosphate ester surfactant, a amidoamine salt of high-molecular polyester acid or the like can be used.
As the alcohol having a boiling point of 178xc2x0 C. or more, either a monohydric or polyhydric alcohol may be used. Examples of the monohydric alcohol include 1-octyl alcohol, 2-octyl alcohol, nonyl alcohol, decyl alcohol, 1-methylcyclohexanol, trimethylcyclohexanol, ethylene glycol monoacetate, diethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monohexyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monovinyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monobutyl ether, ethylene glycol isoamyl ether, ethylene glycol phenyl ether, ethylene glycol benzyl ether, trimethylhexanol, tetrahydrofurfuryl alcohol, cresol, butyl lactate, benzyl alcohol, hydroxyethyl acrylate, phenethyl alcohol, mercaptobutanol, hydroxyethyl methacrylate, hydroxyethyl piperazine, cyclohexanone oxime, hydroxymethoxyallyl benzene, hydroxymethoxybenzaldehyde, hydroxymethylpiperazine, hydroxypropionitrile, hydroxyactonaphthone, hydroxybenzaldehyde, hydroxyactophenone, hydroxybenzimidazole, phenylphenol, hydroxybenzoic acid, hydroxybenzophenone, benzoin, thymol, hydroxymethoxybenzoic acid, hydroxymethylbenzoic acid, hydroxymethylpyrone, hydroxynaphthoic acid, hydroxynaphthoquinone, hydroxynorbomene dicarboxyimide, hydroxyphenyl acetic acid, hydroxyphenyl glycine, hydroxyphthalimide, hydroxypivalic acid neopentyl glycol ester, hydroxypropiophenone, hydroxystearic acid, hydroxysuccinic imide, hydroxytoluic acid, pentaerythritol diacrylate monostearate, and mixtures thereof.
Examples of the polyhydric alcohol include ethylene glycol, propylene glycol, trimethylene glycol, butylene glycol, tetramethylene glycol, pentamethylene glycol, butenediol, hexamethylene glycol, heptanediol, octanediol, nonanediol, decanediol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, glycerin, hexanetriol, heptanetriol, threitol, erythritol, arabitol, xylitol, ribitol, adonitol, glucitol, mannitol, iditol, talitol, galactitol, allitol, perseitol, volemitol and the like.
Examples of the fatty acid amide (4) include acetic amide, lactic amide, propionic amide, valeric amide, hexanoic amide, heptoic amide, octanoic amide, decanoic amide, nonanoic amide, stearic amide, oleic amide, erucic amide and the like.
The photosensitive copper paste of the present invention may further contain a preservation stabilizer such as a polymerization inhibitor, an antioxidant, a dye, a pigment, an antifoaming agent, a surfactant, etc. according to demand.
In the photosensitive copper paste of the present invention, the oxygen content of the copper powder is preferably about 0.8% by weight to 5% by weight.
By setting the oxygen content of the copper powder in the range of about 0.8% to 5% by weight, the occurrence of gelation can be sufficiently suppressed either in a paste state before coating or in a coating state after coating and drying.
The reason for setting the oxygen content in the range of about 0.8% by weight to 5% by weight is that with the oxygen content of the copper powder of less than about 0.8% by weight, the internal layer composed of Cu2O as a main component readily appears in the outermost layer during kneading, for example, with three rolls in the process for producing the photosensitive copper paste, thereby failing to prevent gelation. While with the oxygen content of the copper powder of over about 5% by weight, the rate of volumetric shrinkage is increased in the step of reducing and burning the copper pattern formed by using the photosensitive copper paste of the present invention, thereby easily causing disconnection of the formed copper pattern.
In the photosensitive copper paste of the present invention, the copper powder is coated with a copper oxide by heating the copper powder to room temperature or higher in an oxygen-containing atmosphere.
By heating the copper powder to room temperature or higher in the oxygen-containing atmosphere, the copper powder having the surface coating of copper oxide can efficiently be obtained, in which at least the surface layer having a thickness of about 0.1 xcexcm from the surface is composed of CuO as a main component. Therefore, the present invention can be made effective.
The reason why the method of heating in the oxygen-containing atmosphere is preferable as the method of coating the surface the copper powder with a copper oxide is that this method can easily control the CuO state of the surface of the copper powder and can form a dense CuO film.
For example, the present invention can use a copper powder the surface of which is coated with a Cu oxide by a CuO spray method or an oxidation method using a solution containing an oxidizing agent. However, the CuO spray method or the oxidation method using a solution containing an oxidizing agent cannot easily form a dense CuO film. Therefore, from the viewpoint of sufficient prevention of gelation, the copper powder having the surface coated with a Cu oxide by the method of heating in an oxygen-containing atmosphere is preferably used.
In the photosensitive copper paste of the present invention, the volume fraction of the burning residue remaining after burning is preferably about 30% to 89%. This is because with the volume fraction of less than about 30%, volumetric shrinkage significantly occurs during burning to cause disconnection of the formed copper pattern, while with the volume fraction of over about 89%, the strength of the formed copper pattern (before burning) is significantly decreased to cause breakage of the pattern during burning.
In the present invention, the volume fraction of the burning residue represents the volume fraction of the inorganic components (copper, etc.) remaining after burning and contained in a solid obtained by removing the components (the organic solvent, etc.) from the photosensitive copper paste during drying.
In the photosensitive copper paste of the present invention, the organic binder preferably comprises an acrylic copolymer having carboxyl groups in the side chains. By using the acrylic copolymer having the carboxyl groups in the side chains as the organic binder, it is possible to perform development with water or an alkali aqueous solution while suppressing the occurrence of gelation. Also, the organic binder is useful as a photosensitive organic binder.
With the organic binder comprising the acrylic copolymer having the carboxyl groups in the side chains, the carboxyl groups of the acrylic copolymer easily react with Cu2O in the copper powder. However, even in such a system, gelation can be securely suppressed by using the copper powder having the surface coating composed of CuO as a main component at least in the surface layer having a thickness of about 0.1 xcexcm from the surface.
Examples of the organic binder comprising the acrylic copolymer having carboxyl groups in the side chains can be produced by copolymerizing an unsaturated carboxylic acid and an ethylenic unsaturated compound. Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid, fumaric acid, vinylacetic acid, and anhydrides thereof and the like. Examples of the ethylenic unsaturated compound include acrylic acid esters such as methyl acrylate, ethyl acrylate and the like; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate and the like; fumaric acid esters such as monoethyl fumarate and the like. As the acrylic copolymer, the following copolymers may be used, in which an unsaturated bond is incorporated.
(1) A copolymer obtained by adding an acrylic monomer having a functional group such as an epoxy group or the like, which can react with the carboxyl groups in the side chains of the acrylic copolymer, to the carboxyl groups; and
(2) A copolymer obtained by reacting the acrylic copolymer having epoxy groups introduced in place of the carboxyl groups in the side chains with an unsaturated monocarboxylic acid, and then introducing a saturated or unsaturated polyhydric carboxylic anhydride.
A method of forming a copper pattern of the present invention comprises the steps of applying the photosensitive copper paste of the present invention on a support member, exposing and developing the photosensitive copper paste to form a predetermined copper pattern on the support member, and transferring the copper pattern formed on: the support member to a substrate.
The method of forming a copper pattern of the present invention uses the photosensitive copper paste of the present invention, which comprises the copper powder having the surface coating of copper oxide in which at least the surface layer having a thickness of about 0.1 xcexcm from the surface is composed of CuO as a main component. Therefore, gelation of the photosensitive copper paste and gelation of the coating after drying can be sufficiently suppressed to form a fine copper pattern with high precision.
In the present invention, a substrate is a wide concept including various types of transfer objects, for example, sintered ceramic substrates such as an alumina substrate and the like, and unsintered ceramic green sheets, and the like.
A circuit board of the present invention comprises a circuit formed by forming a predetermined copper pattern by using the photosensitive copper paste of the present invention, and then burning the copper pattern.
The circuit board of the present invention comprises a fine and thick circuit (copper pattern) formed by forming a predetermined copper pattern by using the photosensitive copper paste of the present invention, and then burning the copper pattern, thereby permitting realization of high-density wiring and high-speed signal which cannot be realized by a conventional circuit board.
A method of producing a ceramic multilayer substrate of the present invention comprises the steps of applying the photosensitive copper paste on a support member, exposing and developing the photosensitive copper paste to form a predetermined copper pattern on the support member, transferring the copper pattern formed on the support member to a ceramic green sheet, laminating ceramic green sheets (on each of which the copper pattern was transferred) to form a laminate, and burning the laminate.
The method of producing a ceramic multilayer substrate of the present invention uses the photosensitive copper paste of the present invention, which comprises the copper powder having the surface coating of a copper oxide, at least the surface layer having a thickness of about 0.1 xcexcm from the surface being composed of CuO as a main component. Therefore, gelation of the photosensitive copper paste and gelation of the coating after drying can be sufficiently suppressed to form a fine copper pattern with high precision, thereby obtaining a ceramic rams multilayer substrate having excellent adaptability to higher density wiring and higher speed signals.
The ceramic multilayer substrate of the present invention is produced by the above described method, and comprises a copper pattern which is formed by applying the photosensitive copper paste of the present invention and burning the coating, and which is provided in the substrate or provided in the substrate and on a surface thereof.
The ceramic multilayer substrate of the present invention comprises a fine and thick circuit (copper pattern) formed by forming a predetermined copper pattern on an insulating substrate by using the photosensitive copper paste of the present invention, and then burning the copper pattern, thereby permitting realization of higher-density wiring and higher-speed signal which cannot be realized by a conventional ceramic multilayer substrate.
In the present invention, the possible reason why gelation is suppressed by using the copper powder having the surface coating of a copper oxide in which at least the surface layer having a thickness of about 0.1 xcexcm from the surface is composed of CuO as a main component is the following.
At room temperature in air, the uppermost (outermost) layer of the copper powder in the level of several nm from the surface is coated with a copper oxide composed of CuO as a main component, but the inner region is composed of Cu2O as a main component. At room temperature in air, CuO is more stable than Cu2O, and CuO does not react with the acid functional group in the organic binder, while the Cu2O easily reacts with the acid functional group in the organic binder. Therefore, in producing the photosensitive copper paste by using such copper powder, a thin uppermost layer (the layer composed of CuO as a main component) is separated during kneading (mixing) of the paste, for example, with a three roll mill in the process for producing the photosensitive copper paste, to expose the region (inner layer) composed of Cu2O as a main component to the uppermost layer of the copper powder, thereby causing gelation due to reaction between Cu2O and the acid functional group in the organic binder.
On the other hand, in the present invention using a copper powder having the surface coating of a copper oxide in which at least the surface layer having a thickness of about 0.1 xcexcm from the surface is composed of CuO as a main component, the region composed of CuO as a main component has a large thickness, and thus the region (inner layer) composed of Cu2O as a main component does not appear in the uppermost layer of the copper powder even during kneading of the paste with the three rolls in the process for producing the photosensitive copper paste. Therefore, it is possible to securely suppress the occurrence of gelation due to reaction between Cu2O and the acid functional group in the organic binder.